This invention relates to an illumination system for a document scanning system and, more particularly, to a fluorescent lamp and an associated control system to maintain the illumination profile at preset optimum operational conditions.
Low pressure, mercury vapor fluorescent lamps are used in a variety of lighting applications. Of particular interest, for purposes of the present invention, is the widespread use of fluorescent lamps to illuminate documents being copied by an electrophotographic reproduction machine.
In a conventional mercury fluorescent lamp, an electrical discharge is generated in a mixture of mercury vapor, at low pressure, and a fill gas. The light output from the lamp depends on a number of variables, the most important of which is the mercury vapor pressure inside the lamp tube, and the corresponding cold spot temperature. Prior art techniques for maintaining the mercury vapor pressure at some optimum value are well known in the art, and typically require a temperature-sensitive device such as a thermocouple, thermistor or thermostat to monitor the temperature of the cold spot. A feedback circuit provides closed loop control of a temperature-regulating device to maintain the optimum mercury pressure.
Thus, in a conventional type copier, and under normal operating conditions, it is known how to stabilize a fluorescent lamp output to provide the desired lamp illumination profile at the surface of the photosensitive image plane. However, it has been found that there are some operating conditions which cause transient illumination variations along the lamp axis. These variations create a non-uniform output along the length of the lamp resulting in a non-uniform exposure level at the photosensitive image plane of the reproduction machine resulting in defects in output prints which are derived from developing the exposed latent image. These operating conditions, and their negative impact on lamp operation, are as as follows:
1. When the particular machine in which the lamp is used is in an extended standby period, it has been found that the lamp output varies (drifts) from on optimum profile for the first minute or two of operation following the extended standby period. Any exposure cycle accomplished during this initial start-up time may experience the undesirable axial illumination variation.
2. At initial installation and at lamp replacement, there may be a break-in period lasting from a few days to a week during which the mercury migrates along the lamp until it settles into a relatively stable distribution along, and to some extent within, the fluorescent coating the walls of the lamp. This instability results in the axial illumination variation described above.
3. Installation and usage of equipment in conditions where the ambient temperature is extremely cold, or varies thermally over a wide range can have a negative impact on the lamp temperature stabilization resulting in changes in the lamp illumination profile.
According to a first aspect of the invention, there is provided an apparatus and process in a document and scanning system, for automatically detecting a system standby period of some predetermined duration (for the example given, one hour) and for automatically turning the fluorescent lamp on to full power for approximately two minutes. This periodic lamp energization reduces the undesirable axial illumination variations described in the above situation. According to a second aspect of the present invention the lamp specifications are automatically checked at the end of the 2 minute on interval and, if found to be out of specification, the electronic gain applied to each pixel of a photosensor utilized in the scanning system is automatically readjusted to compensate to the corresponding illumination changes in the lamp. According to a still further aspect of the invention, the machine is available for use in a copying operation during the automatic warmup and calibration/recalibration check process.
The following patents have been identified in a prior art search:
U.S. Pat. No. 3,672,759 to Bauer discloses an illumination control system which compensates for gradual dimming or deterioration of exposure lamps in a photocopying machine. An accumulator keeps a count of all reproductions which is used to control a variable current generator which in turn controls illumination levels. See column 5, line 53 to column 6, line 49.
U.S. Pat. No. 3,679,306 to Du Bois et al. discloses a feedback system for controlling light level in an electrostatic photocopier. A sensor is used to detect illumination and serves as a source of a feedback signal which can compensate for variations in light energy due to lamp aging, dirty optics, etc.
U.S. Pat. No. 4,512,652 to Buck et. al., assigned to Xerox Corporation, discloses a control scheme for changing characteristics of a photoconductive member used in a printing machine. A controller determines charging current used for a photoconductive member by entering a scale factor based on the rest time between copy cycles into a controlling algorithm.
U.S. Pat. No. 4,540,270 to Imanaka et al. discloses an electrophotographic copying machine which has a timing means which is operated upon the completion of a copying operation for counting a time during which a pre-copy process is started to stabilize the property of a photoreceptor.
U.S. Pat. No. 4,739,367 to Watanabe et al. discloses an image forming apparatus having a control means for effecting control of a preliminary processing of image formation. A timer is provided for timing the period of time passed since an image transfer occurred. A control means effects the agitation of a developing means based on the time period.
U.S. Pat. No. 4,508,446 to Imai discloses a reproduction process control method for a copier which maintains the quality of copy images uniform even during temperature changes. Various combinations of variables including charging amount, exposure time, and bias voltage are changed depending upon photosensitive member temperature as compared with a reference level to maintain the quality.
U.S. Pat. No. 4,183,657 to Ernst et al. discloses a system for checking the copy quality within an image area of an electrophotographic machine. During a test cycle, variables such as toner concentration, image voltage and low reflectance are checked. The checks may be done during a shutdown cycle or during small production runs.
U.S. Pat. No. 4,372,672 to Pries discloses a self-triggering quality control sensor which can work with a system as in the '657 patent. A photosensor senses an untoned reflectance and stores a signal in a sample circuit. The circuit triggers itself when a transducer produced signal varies from a sample area which negates the need for testing to be controlled by the copy machine.
And, more particularly, the invention relates to a method for compensating for illumination variations along the length of a linear illumination lamp utilized in a document scanning system including the steps of:
(a) determining that the lamp current has been off for a cumulative time period exceeding a preset value T.sub.1.
(b) energizing the lamp and maintaining the lamp in on condition for a period of time T.sub.2 and,
(c) initiating a diagnostic routine following said time period T.sub.2 to determine whether the lamp output has varied sufficiently from predetermined specifications to require compensation.